The crystallographic texture of a plate used as a sputtering target is of great importance to the sputtering performance, particularly to the uniformity of thickness of the thin films deposited on substrates. Only a plate with uniform texture throughout its volume will give optimum performance, and users rely on a steady supply of plates with similar texture. However, the manufacture of plates by existing, state-of-the-art, methods does not produce uniform texture.
A tantalum plate produced from an electron-beam-melted ingot by conventional processing (1980's technology), i.e. side-forging, rolling and annealing, has a banded texture, a legacy of the large grains formed during solidification. It also has a through-thickness texture gradient, caused by variation of the shear strain (through the thickness) induced during rolling. It may also show incomplete recrystallization, and grain-size banding.
Various advances and improvements in processing tantalum ingots have been published:
Pokross, in ‘Controlling the Texture of Tantalum Plate’, JOM October 1989, and Clark et al. in a series of 3 papers in Metallurgical Transactions A in 1991 and 1992 described the value of bi-directional rolling (also known as cross-rolling) and multiple anneals.
Michaluk et al., in U.S. Pat. Nos. 6,348,113 and 6,893,513, discloses methods of preparing tantalum plates that do not achieve high levels of uniformity.
Jepson et al, in U.S. patent application Ser. No. 10/079,286, discloses refractory metal plates having a reduction in texture banding severity achieved by introducing upset/forge-back/anneal sequences.
Turner, in U.S. Pat. No. 6,331,233, discloses a process for preparing tantalum plates which reduces banding severity, but which results in a rather strong through-thickness texture gradient.
Shah and Segal, in U.S. Pat. No. 6,348,139, disclose the use of a low-friction interfacial layer in upset forging to achieve a more uniform strain. The use of low-friction interfacial layers in upset forging of other materials has long been known (e.g., Morra and Jepson, Superalloys 718, 625, 706 Conference, 1997).
Field et al., “Microstructural Development in Asymmetric Processing of Tantalum Plate” in Journal of Electronic Materials, Vol 34, No 12, 2005, introduced the concept of asymmetric processing to achieve shear strain throughout the thickness of a plate.
Kumar et al., in U.S. Pat. No. 6,521,173, disclose the manufacture of metal powder suitable for consolidation into plates for sputtering. Although powder consolidated by hot isostatic pressing has a random, perfectly uniform texture, some texture, including a through-thickness texture gradient, develops when the block of consolidated powder is rolled to plate and annealed.
Koenigsmann and Gilman, in U.S. Pat. Nos. 6,770,154 and 7,081,148, disclose tantalum sputtering targets made by powder-metallurgy with particular proportions of various grain orientations and an absence of visible banding. Targets made in accordance with these patents but involving a rolling step would have a through-thickness texture gradient.
Advances have been made in measuring texture, and the measurements can be used in such a way that the uniformity of texture can be described quantitatively. The EBSD (electron back-scatter diffraction) technique measures texture grain-by-grain (whereas X-ray diffraction, the only technique available until the early 2000's, only measured an aggregate over the area irradiated, which covered many grains), and EBSD equipment which can cover the full thickness of the plate in a reasonable time is now available. Methods of quantifying texture uniformity were described both by Michaluk, in U.S. Pat. No. 6,348,113 and by Jepson in U.S. patent application Ser. No. 10/079,286, but both of these methods were rudimentary and unsatisfactory. Another method, somewhat improved, was described by Michaluk et al. in JOM, March 2002. Presently, an ASTM standard for quantification of texture is being drafted, following initiatives of Sutliff and Jepson (unpublished), and the proposed ASTM standard method will be used in the present application to describe the uniformity of texture.
Three factors must be calculated and used to give an overall view of the non-uniformity of texture within a plate:
a) Through-thickness gradient
b) Banding severity
c) Variation across a plate.
If the same measurements of texture are made for a multiplicity of plates made by the same process, the stability of the process from plate to plate can also be estimated.
The rate of sputtering from a grain in the target depends on the orientation of the crystal planes of that grain relative to the surface (ref. Zhang et al, “Effect of Grain Orientation on Tantalum Magnetron Sputtering Yield”, J. Vac. Sci. Technol. A 24(4), July/August 2006). Also, certain crystallographic directions are preferred directions of flight of the sputtered atoms (ref. Wickersham et al, “Measurement of Angular Emission Trajectories for Magnetron-Sputtered Tantalum”, Journal of Electronic Materials, Vol 34, No 12, 2005). The grains of a sputtering target are so small (typically 50-100 μm diameter) that the orientation of any individual grain has no significant effect. However, the texture of an area of the sputtered surface (an area roughly 5 cm to 10 cm diameter) can have a significant effect. Thus, if the texture of one area on the surface of a target is different from the texture of any other area, the thickness of the film produced is unlikely to be uniform over the whole substrate. Also, if the texture of a surface area is different from that of the same area at some depth into the target plate, the thickness of the film produced on a later substrate (after the target is used, or eroded, to that depth) is likely to be different from that produced on the first substrate.
So long as the texture of one area, then, is similar to that of any other, it is not important what that texture is. In other words, a target plate in which every grain has a 111 orientation parallel to the plate normal direction (ND) is no better and no worse than one in which every grain has a 100 orientation parallel to ND, or than one which consists of a mix of 100, 111 and other grains, so long as the proportions of the mix remain constant from area to area.
Uniformity of film thickness is of major importance. In integrated circuits, several hundred of which are created simultaneously on a silicon wafer, for example, too thin a film at one point will not provide an adequate diffusion barrier, and too thick a film at another point will block a via or trench, or, if in an area from which it should be removed in a later step, will not be removable. If the thickness of the film deposited is not within the range specified by the designer, the device will not be fit for service, and the total cost of manufacture up to the point of test is lost, since no repair or rework is normally possible.
If the target does not have uniform texture, and thus does not provide a predictable, uniform sputtering rate, it is impossible, in state-of-the-art sputtering equipment, to control the variation of thickness from one point on the substrate to another. Partial, but not total, control of variation of thickness from substrate to substrate, and from target to target, is possible using test-pieces. Use of test-pieces, however, is time-consuming and costly.
With targets made according to the prior art, the non-uniformity of texture found in the target plate caused unpredictability in the sputtering rate (defined as the average number of tantalum atoms sputtered off the target per impinging argon ion), or a change in the sputtering rate as the target was used. Variations of sputtering rate cause variation of the thickness of the film produced from point to point on the substrate, and also cause variation of average thickness of the film produced on the substrate from substrate to substrate, and from target to target.